From: Brian Candler <b.candler@...>
Date: 2009-07-21T22:35:27+09:00
Subject: Re: Mean method

Älphä Blüë wrote:
> http://pastie.org/553287

Are you posting here because you want comments/suggestions? If so, 
here's my 2c.

(1) Passing enumerated values 1,2,3 is not very Ruby-like. If you want 
to go this way, then I'd suggest symbols: :arithmetic, :geometric, 
:harmonic

(2) However, IMO it's also code smell if you write something like

  case A
  when X
    .. do one thing
  when Y
    .. do another thing
  ... etc
  end

There's usually a better way to ask directly for what you want.

Suggestions:

(a) add some methods directly to class Array, in the same way as 'sum'

class Array
  def arithmetic_mean
    self.sum / self.size
  end

  def geometric_mean
    ... etc
  end
end

puts [1,2,3,4,5].arithmetic_mean

This is perfectly fine within your own apps, but modifying core classes 
may not be acceptable in a shared library (depending on who your target 
audience is)

(b) wrap an array with an object which does the calculation

class Mean
  def initialize(arr)
    @arr = arr
  end

  def arithmetic
    @arr.sum / @arr.size
  end

  ... etc
end

puts Mean([1,2,3,4,5]).arithmetic

Note that creating an object like this is a cheap operation in Ruby. 
You're not copying the array elements, just a reference to the array 
itself.

(3) It is considered extremely bad form to write

  if !an_array.is_a? Array: raise "mean(#{an_array}) error: #{an_array} 
is not an array." end

You should simply leave this out. This lets the user pass in any object 
which responds to the particular methods you call on it (each, sum, 
size). This is the "duck typing" principle - if someone wants to create 
an object which behaves like an Array, even though it is not actually an 
Array, why should you forbid them from using it?

If they pass in an incompatible object, Ruby will generate an exception 
for you anyway. Admittedly it might not be quite as obvious what's 
happening, but it will typically be something along the lines of

  NoMethodError: object does not respond to 'sum'

which tells you exactly what the problem is.

(4) You can rewrite the following using inject:

      an_array.each do |a|
        n_sum += (1.0 / a)
      end

(5) If I were you, I would rewrite this not to use sum and size, but 
just to iterate over the collection using 'each' (or something which in 
turn calls 'each', like 'inject'), calculating the total and count as 
you go. This would allow your code to work on any object which is 
Enumerable, such as

   (1..5).arithmetic_mean

Regards,

Brian.
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